In a general medium-speed gas engine, an air supplied through an air supply path (supply air) and a fuel gas supplied from a fuel supply path are mixed and then supplied to a main combustion chamber of an engine so that combustion operation is performed to obtain driving force. There has been also known a precombustion-chamber pilot-ignition type gas engine, in which a precombustion chamber that communicates with the main combustion chamber is disposed on a cylinder head. A liquid fuel such as the gas oil is injected into the air flow formed inside the precombustion chamber from a fuel injection valve to be ignited and combusted. The ignition flame thereof is injected from an injection opening of the precombustion chamber into a mixed air of the air and fuel gas introduced into the main combustion chamber by opening an air supply valve, and thereby the mixed air is combusted.
Further, a spark-ignition type gas engine has been also known, in which a fuel gas for ignition is supplied to a precombustion chamber provided for the cylinder head to produce a mixed air of high ignition performance. The mixed air is ignited and combusted by a spark plug. The ignition flame is injected into a mixed gas of the air and the fuel gas introduced into the main combustion chamber by opening an air supply valve, and thereby the mixed gas is combusted.
In the above gas engines, the flow rates of the fuel and air supplied to the combustion chamber are appropriately controlled so that a required engine output is achieved. Also, the air-to-fuel ratio is controlled so as to enable stable operation while avoiding knocking or misfire. However, abnormal combustion (knocking) or misfire is likely to occur when there is a trouble such as abrasion and malfunction of a component in the fuel supplying system or the ignition system.
For instance, a misfire occurs in all cylinders or in a particular cylinder when a required fuel gas is not supplied due to malfunction of a fuel flow rate control valve that supplies the fuel gas to the air supply path from the fuel supply path, or when there is a trouble of the ignition device such as malfunction of a spark plug or a control valve that supplies a liquid fuel such as the gas oil to the precombustion chamber.
As an example of the technique for detecting a misfire of a gas engine to forcibly stop the engine is proposed in JP2001-12292 (Patent Document 1) and JP2007-170405 (Patent Document 2).
Patent Document 1 discloses, in the abstract and paragraphs 0063 to 0067 etc, detecting an exhaust pressure waveform of an engine including a plurality of cylinders while distinguishing each cylinder, and calculating the area of the exhaust pressure waveform in a predetermined crank angle range for each cylinder. The calculated result is compared to a moving average value of the last predetermined cycle. The ratio to the average value of the waveform area is compared to a reference value to determine a misfire. When a misfire is determined, the engine is forced to stop.
Further, Patent Document 2 discloses, in paragraphs 0031 and 0032 etc, calculating a pressure difference ΔP (ΔP=P−Pb) from the reference pressure Pb before starting compression including the air intake pressure in accordance with a crank angle based on an in-cylinder pressure detection value inputted from an in-cylinder pressure detector and a crank angle detection value inputted from the crank angle detector, and determining occurrence of a misfire in the combustion chamber when the maximum in-cylinder pressure ratio (ΔPp/ΔP0), which is a ratio of the pressure difference ΔPp between the maximum in-cylinder pressure Pp and the reference pressure, to the pressure difference ΔP0 of one or more arbitrary points in the compression stroke, becomes not greater than the set minimum misfire-tolerance pressure ratio Pn. It also discloses shutting off the fuel injection of the cylinder where a misfire has occurred upon the number of cycles reaching the tolerance thr continuation of cycles.